Human hair fiber is produced in the hair follicles, which are composed of different particular epithelium and connective tissues and are one of the most active tissues in human body. It is well known that hair fiber is composed mainly of keratin-intermediate filaments (KIFs) and keratin-associated protein (KAPs), that KIFs are further classified into Type I and Type II, and forms a gene cluster on human chromosome 12 and 17 (Cytogenet. Cell Genet. 48, 148–151, 1988, Exp. Cell Res. 220, 357–362, 1995, Mol. Cell. Biol. 8, 722–736, 1988, Cytogenet. Cell Genet. 77, 169–174, 1997, Hum. Genet. 82, 109–112, 1989, Cytogenet. CellGenet. 57, 33–38, 1991, Genomics 24, 502–508, 1994). On the other hand, from the studies of animal other than human, it is reported that KAPs (keratin-associated protein) play a role as matrix embedding the space between KIFs and they are roughly classified into 3 groups based on their amino acid composition, that is ultra-high sulfur protein (cysteine content 30% or more), high sulfur protein (cysteine content 30% or less), and high Glycine/Tyrosine protein (glycine and tyrosine content 35–60%). As for KAPs, many cDNA are isolated from mice, rats, rabbits, sheep and the like, and for the present, proteins belonging to 17 sub-families have been identified (Powell, B. C., and Rogers, G. E. The role of keratin proteins and their genes in the growth, structure and properties of hair. In Formation and structure of human hair (ed. Jolles, P., Zahn, H., and Hocker, H.), pp. 59–148, Birkhauser Verlag, Basel., 1997; Genomics 54, 437–442, 1998; J. Invest. Dermatol. 111, 128–132, 1998; J. Invest. Dermatol. 111, 804–809, 1998; Mech. Dev. 86, 193–196, 1999; Development 128, 1547–1558, 2001; J. Biol. Chem. 276, 19440–19451, 2001). On the contrary, studies on human KAP have just begun.
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